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What's the farthest distance a star visible to the naked eye can still be seen?

by eehiram
Tags: distance, farthest, naked, star, visible
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eehiram
#1
Jan11-07, 09:48 AM
P: 121
I was wondering how far away the stars are at night that can be seen by the naked eye. I would venture to guess that the galaxies that are billions of light years away must be seen with a high tech telescope.

o| Hiram
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DaveC426913
#2
Jan11-07, 10:12 AM
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2.5 million light years - the distance to the Andromeda galaxy. It is visible to the naked eye.

Now, they're only visible as a collection, so that may disqualify them in your judgement.

As for the farthest individually distinguishable star, that's a tricky question. There are factors:
- an observer with good eyesight in perfect viewing conditions can see down to about Mag 6.5.
- the distance to stars is only one factor that affects its visibility. Size is a another major factor. Larger stars appear brighter, thus can be seen from farther away.

http://www.wonderquest.com/naked-eye.htm

"The farthest star that we can see under the best of conditions is probably about 4,075 light years away."
eehiram
#3
Jan11-07, 11:02 AM
P: 121
In point of fact, I did not mean "star" as in single star, but "star" as one would refer to a light in the night sky. Thus I include galaxies.

I just wanted to know how far away and far back in time I am looking at at night.

o| Hiram

russ_watters
#4
Jan11-07, 06:29 PM
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What's the farthest distance a star visible to the naked eye can still be seen?

Well, more than 99% of what you see with your naked eye is stars in our galaxy (or planets), so the fact that Andromeda is 2.5 million light years away only applies if you are looking at Andromeda or one of the small handful of other galaxies visible to the naked eye (probably only 2, really, and only Andromeda in the northern hemisphere). The rest of what you see is stars in our galaxy and they are all less than about 4,000 light years. (as Dave said)
eehiram
#5
Jan11-07, 11:25 PM
P: 121
I guess that settles it then. Now I will no longer say that we are looking billions of years into the past when we look up at night.

o| Hiram
DaveC426913
#6
Jan12-07, 12:34 AM
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If you look toward the Andromeda galaxy with the naked eye, you are looking 2.5 million years into the past.
HallsofIvy
#7
Jan12-07, 09:21 AM
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Quote Quote by eehiram View Post
I guess that settles it then. Now I will no longer say that we are looking billions of years into the past when we look up at night.

o| Hiram
Unless you just like saying "billyuns and billyuns"!
eehiram
#8
Jan12-07, 01:19 PM
P: 121
Now that we've straightened that out, what would you all say is the likelihood that the stars of the Andromeda have already fused themselves out? Can we say that they may have? It's been 2.5 million years since they emitted the light that we now see.
DaveC426913
#9
Jan12-07, 05:27 PM
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Our sun, a typical main sequence star, will last another 5 billion years.

2 million years in a main sequence star's lifetime is about 2 years in a human's lifetime.
chroot
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Jan12-07, 05:50 PM
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It's safe to say that some of the stars in the Andromeda galaxy have died in the last few million years, though we still see the light they emitted a few million years ago. On the other hand, as DaveC points out, the vast majority of stars are in their "adult" years, the main sequence, and haven't changed much at all in that same length of time.

- Warren
eehiram
#11
Jan13-07, 12:37 AM
P: 121
Now that helps. I was going to ask about when the Andromeda Galaxy would have started out. I take it "adult" cycle is approximate in it's intent.

That means all the lights in the sky (except for some of the Andromeda stars, as you mentioned) are still there at their locations for now. Cool.
chroot
#12
Jan13-07, 01:56 PM
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The main sequence is rather well-defined, but its length depends upon the mass of the star. Big stars die faster than small ones.

- Warren
EvilEyeMonster
#13
Jan13-07, 06:23 PM
P: 3
The sun.

We have no way of knowing when we look, whether the ones we are looking at are still there at the time.
russ_watters
#14
Jan13-07, 07:28 PM
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Uh, did you read any of the other posts in the thread?
EvilEyeMonster
#15
Jan13-07, 07:48 PM
P: 3
Quote Quote by russ_watters View Post
Uh, did you read any of the other posts in the thread?

Yes I did.

What bothers me about the question is that it ask how far can you see.

You can see only right in front of you. That is all.

You know from your own physics lab, that the closer you get to the speed of light, the slower you age.

This means that a light-wave traveling AT the speed of light is precicely the same age it was when it left the source, and finally entered your eye.

THEREFORE any star we view with the eye, or even a telescope... is in inherently just as new as it was when it was created.

I just mentioned our sun (as a star) because even though the light is apx. 8 minutes old from our point of view... it is actually just as new as it was when it left.

AGAIN... at the SPEED of LIGHT.... time and space are negligeable...

So therefore... light that gets to us from anywhere in the universe is the SAME age.



(edit) So I guess the answer to the original question is.... as far as you look.
chroot
#16
Jan13-07, 08:02 PM
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Quote Quote by EvilEyeMonster View Post
I just mentioned our sun (as a star) because even though the light is apx. 8 minutes old from our point of view... it is actually just as new as it was when it left.
This is not what the original poster meant. Even though photons don't "age," we see the Sun as it was 8 minutes ago, and the Andromeda galaxy as it was millions of years ago.

Certainly, the stars in the Andromeda galaxy can be seen, and they're more distant than the Sun -- so the Sun cannot be the answer to the original question.

- Warren
eehiram
#17
Jan13-07, 11:12 PM
P: 121
I'm a little confused now: What does it mean to say that the photons are the same age when they enter into the human eye on Earth as when they were emitted from the distant stars? How does that reflect on time passing for the stars themselves in the meantime? Is this supposed to mean that the sight of the stars is still "fresh"? That's interesting, but that wasn't my original question itself.
DaveC426913
#18
Jan14-07, 12:29 PM
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EvilEyeMonster may mean well, but he is confused and using fancy footwork to hide it.

"THEREFORE any star we view with the eye, or even a telescope... is in inherently just as new as it was when it was created."

What he just said (whether or not it's what he meant to say) is that all stars we see, including our own Sun, appear to us as if they have just been born. This is of course, nonsense.




What he is getting at is that photons do not experience time. They do not age. If we could see what the photons sees (we can't, even in theory), we would see our emission from the Andromeda galaxy, and arrival here on Earth* as simultaneous.

This part of what he says is true, though I fail to see the relevance.


What EEM is missing is that the age of the light itself is irrelevant, what we are interested in is the age of the image.


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